Trump regrets hiring Attorney General Jeff Sessions
US President Donald Trump has said he would never have appointed Jeff Sessions if he had known the attorney general was going to recuse himself from leading a Russia investigation.
Mr Trump told the New York Times the actions of Mr Sessions had been "very unfair to the president".
Mr Sessions recused himself after admitting meeting Russia's ambassador.
He said on Thursday he would not resign and he would continue running the Justice Department effectively.
"I plan to continue to do so as long as that is appropriate," he said.
The president also accused Mr Sessions of giving "some bad answers" at his confirmation hearing performance.
The price of loyalty - Anthony Zurcher, BBC News, Washington
With Donald Trump, loyalty will only get you so far.
Mr Sessions was the earliest and most enthusiastic of Mr Trump's top-tier political supporters, and he was rewarded with a plum Cabinet appointment. Now, however, that position of power appears not quite as golden a prize.
While the former Alabama senator has toiled to implement the president's agenda as attorney general, Mr Trump personally blames him for the ongoing independent counsel investigation that has bedevilled his presidency.
The irony is that while Mr Trump views Mr Sessions's recusal from the Russia probe as a betrayal, the attorney general made clear during his confirmation hearings that he would likely do just that if he were implicated in an investigation that had not yet begun in earnest.
It was only later that then-FBI Director James Comey - himself a target of the president's scorn - revealed the Trump campaign itself was under the microscope.
Now the president has made clear that Mr Sessions lacks his full confidence. While the attorney general says he loves his job and plans to keep it, how secure can his position be when his boss lobs bomb after bomb his way from the White House?
Mr Sessions would have headed the justice department's investigation into alleged Russian interference in the US presidential election. Congress is also conducting inquiries.
His recusal ultimately led to the appointment of special counsel Robert Mueller to lead the investigation.
The Times interview reflects the anger the president feels at this development.
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He said: "A special counsel should never have been appointed in this case... Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job, and I would have picked somebody else."
Mr Trump said Mr Sessions had given him "zero" notice of the recusal.
"How do you take a job and then recuse yourself? If he would have recused himself before the job, I would have said, 'Thanks, Jeff, but I can't, you know, I'm not going to take you.' It's extremely unfair, and that's a mild word, to the president."
'I didn't do anything wrong'
Mr Trump then reflected on the performance of Mr Sessions at his Senate confirmation hearing in January at which he denied meeting any Russians. He later revealed he had met Russian ambassador Sergey Kislyak.
Mr Trump said: "Jeff Sessions gave some bad answers... He gave some answers that were simple questions and should have been simple answers, but they weren't."
The president suggested the justice department's Russia investigation was rife with conflicts of interest, not least that Mr Mueller had wanted to replace James Comey, who Mr Trump had sacked as FBI director.
"There were many other conflicts that I haven't said, but I will at some point," Mr Trump said.
Mr Trump warned Mr Mueller about straying too far from his remit but again said he did not think he was personally being investigated.
"I don't think we're under investigation," Mr Trump said. "I'm not under investigation. For what? I didn't do anything wrong."
US media have reported that Mr Mueller is investigating Mr Trump for possible obstruction of justice, both in the firing of Mr Comey and over whether Mr Trump tried to end an inquiry into sacked national security adviser Michael Flynn.
Associated Press news agency quoted a Trump adviser as saying that the president's comments did not mean he was going to sack the attorney general, but the adviser questioned whether such a public dressing-down might prompt him to quit.